Fuel cells are used to produce electricity when supplied with fuels containing hydrogen and an oxidant such as air. A typical fuel cell includes an ion conductive electrolyte layer sandwiched between a cathode layer and an anode layer. There are several different types of fuel cells known in the art, one of which is a solid oxide fuel cell (SOFC). SOFCs are regarded as a highly efficient electrical power generator that produces high power density with fuel flexibility and are used in Auxiliary Power Units (APU) to convert hydrocarbon fuels into electrical energy to provide electrical power for stationary or on-board vehicles.
In a typical SOFC, air is passed over the surface of the cathode layer and a reformate hydrocarbon fuel is passed over the surface of the anode layer opposite that of the cathode layer. Oxygen ions from the air diffuse from the cathode layer through the dense electrolyte to the anode layer in which the oxygen ions reacts with the hydrogen and carbon monoxide in the fuel, forming water and carbon dioxide; thereby, creating an electrical potential between the anode layer and the cathode layer. The electrical potential between the anode layer and the cathode layer is typically about 1 volt and power around 1 W/cm2. Multiple SOFCs are stacked in series to form a SOFC stack having sufficient power output for commercial applications.
The anode acts as a catalyst for the oxidation of hydrocarbon fuels and has sufficient porosity to allow the transportation of the fuel to and the products of fuel oxidation away from the anode/electrolyte interface, where the fuel oxidation reaction takes place. The anode of a typical SOFC is typically formed of a nickel/yttria-stabilized zirconia (Ni/YSZ) composition in which the nickel is in the form of nickel oxide (NiO). The use of nickel in the anode is desirable for its abilities to be a catalyst for fuel oxidation and current conductor. After the SOFC stack is initially assembled from a plurality of SOFCs, a fuel gas comprising primarily of hydrogen gas is passed over the anodes of the SOFCs at elevated temperatures to reduce the NiO in the anodes into substantially metallic nickel (Ni).
SOFC stacks are typically operated at above 700° C. and the nickel in the anode remains in its reduced form Ni due to the continuous supply of primarily hydrogen fuel gas. However, if the supply of fuel gas is lost during a controls upset, malfunction, or sudden fuel cut-off, the Ni in the anode of the SOFC may undergo a re-oxidation, where the Ni reacts with the oxygen in the air diffused from the cathode layer or introduced into the anode chamber to form NiO at temperatures above approximately 350° C. The formation of NiO in the microstructure of the anode results in volumetric expansion of the anode layer, which exerts stress on the overall SOFC structure. Repeated nickel oxidation and reduction may cause delamination or cracking of the electrolyte of the SOFC.
In a laboratory setting, the SOFC stack may be protected from re-oxidation using a supply of reducing gas, which is typically a dilute mixture of hydrogen in nitrogen gas. This can be used to purge the anode chamber during SOFC shutdown or standby conditions to prevent anode re-oxidation. A typical SOFC stack requires usually between four to twelve hours cooling from its operating temperature to a temperature below which there is no significant damage to the anode material can occur. During this time, it will require a large amount of purging gas and frequent bottle changes to meet the reducing gas consumption demand; therefore using compressed reducing gas system on a mobile system, especially onboard a vehicle, is impractical.
There is a need for a system to protect the integrity of the SOFC during shutdown operation of the SOFC stack in an APU onboard a vehicle. There is a further need for a system to prevent the oxidation of nickel in the anode layer of the SOFC during periods of prolong shut down. There is still a further need for this system to be portable and economical to install and operate.